1. Field of the Invention
The present invention relates to an apparatus for measuring compressive and tensile forces over extended ranges of force loads utilizing freely interchangeable pre-calibrated force sensors (load cells) of various capacities.
2. Description of the Prior Art
There is no physical phenomenon which directly converts a force into an electrical signal. Therefore, in a force transducer, the force is first converted into a strain (deflection) in an elastic material and in turn this strain is converted into an electrical signal. Utilizing this concept, current strain gage technology provides a strain gage load cell widely used in force measuring and weighing instruments. The load cell is a precisely machined metal strain member to which electronic strain gages are cemented. When force is applied to the metal member, the member is caused to deflect and the electrical resistance of the strain gage changes, which makes it possible to relate the strain (deflection) to the applied force.
In prior art devices, a Wheatstone Bridge circuit, of which the strain gage is a part, is used whereby the output signal of the strain gage load cell is processed by an electronic circuit--detected, amplified and converted from analog to digital format--and visually displayed in units of force on the electronic (LCD or LED) display screen. Such prior art devices, commonly referred to as digital electronic force gages, were first introduced in the 1960's. They generally comprise a metal or plastic housing containing an electronic strain gage force sensor (load cell) dedicated or matched in force capacity to the individual force gage (dynamometer) and an electronic control and processing circuit (printed circuit board) with a digital electronic display (LCD or LED) mounted integrally and battery for power.
The prior digital electronic force gages utilize an electronic strain gage load cell individually calibrated and permanently mated to one electronic control and processing circuit and digital electronic display. The electronic circuit is then adjusted to accommodate the coarse tolerance by adjusting a potentiometer while calibrating the electronic force measuring device with dead weights. Therefore, it is only necessary to control the tolerance of the strain gage load cell output--expressed in millivolts/volts of excitation voltage--to a relatively coarse 5% or more. Because the tolerance of the strain gage load cell is relatively coarse, the cost of manufacture of the strain gage load cell is lower and thus the cost of manufacture of the prior art devices is more economical, but requires the permanent mating of a particular strain gage load cell to a particular electronic control and processing circuit.
Since the accuracy of force measuring instruments is controlled to and expressed as a percentage of the full scale capacity of the particular force gage, the need to change from a force gage with one designated capacity to another force gage with another capacity is apparent as follows:
Force Gage Number 1:
Capacity: 100 pounds force (lbf.) PA1 Accuracy: .+-.0.2% of full scale PA1 Capacity: 10 pounds force (lbf.) PA1 Accuracy: .+-.0.2% of full scale
Maximum allowable error: 100.times.0.002=.+-.0.2 lbf.
Force Gage Number 2:
Maximum allowable error: 10.times.0.002=.+-.0.02 lbf.
Thus, when measuring forces of 10 lbf. or less, the obvious choice of force gages would generally be Force Gage Number 2 with a maximum possible error of .+-.0.02 lbf. versus Force Gage Number 1 with a maximum possible error of .+-.0.2 lbf., which is 10 times greater error.
The dedicated or permanently matched system has the drawback that, when a higher or lower capacity force gage is required to enhance (improve) readability and/or accuracy, the user must acquire an additional complete force gage. When several force gages of various capacities or measuring ranges are required, the physical size and weight becomes a factor for transporting and storing the several instruments. More importantly, with the acquisition of additional force gages, the cost includes all the components of a complete force gage. With each purchase the user must acquire another redundant electronic control and display circuit, which is the most costly component of the force gage.
Another "prior art" digital; electronic force gage generally utilizes all the same component parts as other prior art force gages, but the strain gage load cell is mounted externally to the force gage housing and remotely on an electrical cable to facilitate reading force measurements at a distance from the forces being measured. The force gage can be utilized as either hand-held or mounted on a test stand or test fixture. This remote load cell force gage is either (1) dedicated, permanently matched force sensor to the electronic control and display circuit, or (2) interchangeable, controlled output load cell allowing any one of several remote load cells to be attached to the force gage for use without calibration on site with dead weights. The force measuring capabilities of this remote force gage are generally limited to unique applications where the remote force measuring feature is needed, i.e., the remote load cell may be inserted between two surfaces exerting compression forces or between two objects moving apart and exerting tensile forces. In these applications, the force gage with remote dedicated or interchangeable load cells is very useful, but is limited by its remote load cell configuration.
The present invention is designed to overcome the above limitations that are attendant upon the use of "prior art" devices, and toward this end, it contemplates the provision of a novel method and apparatus for measuring force quantities, which utilizes a self contained digital electronic force gage with interchangeable strain gage load cells, in the form of force cell modules, of various force capacities or measuring ranges.
It is an object to provide such a device which eliminates the redundancy in force gages, that requires an additional electronic control and display circuit for each force capacity or force measuring range.
It is also an object to provide such a device which has precalibrated interchangeable force cell modules of various force measuring ranges that readily attach to the force display module, but do not require calibration to complete the mating of the force sensor (load cell) to the electronic control and display circuit and battery (the force display module).
Still another object is to provide such a device which can be completely self-contained and yet has interchangeable force sensor modules that become integral to the complete assembly to become a force gage of another force capacity or measuring range.
Still another object is to provide such a device with significant financial savings for the end user, in that the same technical measuring result, the basic intended use of such an instrument, is the same at lower overall costs when there is a need for multiple force measuring ranges.
A further object is to provide, conversely, a device that offers interchangeability of the electronic control and display circuit and battery (the force display module) to the end that repair of the force gage is facilitated by the end user, e.g. the cost of having both force display modules and force cell modules as stand-by spares is significantly less than the cost of complete spare force gages.
Another object is to provide such a device which is generally compatible in physical size, form and configuration with "prior art" devices, to be readily adaptable for the same use without disadvantage.
It is a general aim of the invention to provide such a device which may be readily and economically fabricated and will have long life in operation and significantly greater flexibility in use.